Nanosheet devices with different types of work function metals

ABSTRACT

A technique relates to a semiconductor device. A first work function metal is in first stack and second stacks, each having nanowires separated by the first work function metal. A mask is on the first stack such that the first work function metal in the first stack is protected while the first work function metal in the second stack is exposed. The mask is undercut by removing a portion of first work function metal in first stack, leaving a gap. A plug is formed in the gap underneath the mask so as to protect the first work function metal in first stack. First work function metal in the second stack is removed, thereby removing the first work function metal from in between the nanowires of the second stack. The mask and plug are removed from first stack. A second work function metal is formed on first and second stacks.

DOMESTIC PRIORITY

This application is a divisional of U.S. patent application Ser. No.15/938,522, filed Mar. 28, 2018, the disclosure of which is incorporatedby reference herein in its entirety.

BACKGROUND

The present invention generally relates to fabrication methods andresulting structures for semiconductor devices, and more specifically,to forming nanosheet devices with different types of work functionmetals.

The MOSFET is a transistor used for amplifying or switching electronicsignals. The MOSFET has a source, a drain, and a metal gate electrode.The metal gate is electrically insulated from the main semiconductorn-channel or p-channel by a thin layer of insulating material, forexample, silicon dioxide or glass, which makes the input resistance ofthe MOSFET relatively high. The gate voltage controls whether thecurrent path from the source to the drain is an open circuit (“off”) ora resistive path (“on”).

N-type field effect transistors (NFET) and p-type field effecttransistors (PFET) are two types of complementary MOSFETs. The NFETincludes n-doped source and drain junctions and uses electrons as thecurrent carriers. The PFET includes p-doped source and drain junctionsand uses holes as the current carriers.

The nanowire or nanosheet MOSFET is a type of MOSFET that uses multiplestacked nanowires/nanosheets to form multiple channel regions. The gateregions of a nanosheet MOSFET are formed by wrapping gate stackmaterials around the multiple nanowire/nanosheet channels. Thisconfiguration is known as a gate-all-around (GAA) FET structure. Thenanowire/nanosheet MOSFET device mitigates the effects of short channelsand reduces drain-induced barrier lowering.

SUMMARY

Embodiments of the invention are directed to a semiconductor device. Anon-limiting example of the semiconductor device includes a first stackhaving a first two or more nanowires, a first work function metal on thefirst two or more nanowires, and a second work function metal on thefirst work function metal, where the first two or more nanowires areseparated by the first work function metal. The semiconductor deviceincludes a second stack having a second two or more nanowires and thesecond work function metal, where the second two or more nanowires areseparated by the second work function metal.

Embodiments of the invention are directed to a method for fabricating asemiconductor device. A non-limiting example of the method includesforming a first work function metal in a first stack and a second stack,the first stack and the second stack each having two or more nanowiresseparated by the first work function metal. The method includes forminga mask on the first stack such that the first work function metal in thefirst stack is protected while the first work function metal in thesecond stack is exposed, and undercutting the mask by removing a portionof the first work function metal in the first stack, such that a gapremains at a location where the portion is removed. Also, the methodincludes forming a plug in the gap underneath the mask at the locationso as to protect the first work function metal in the first stack, andremoving the first work function metal in the second stack, therebyremoving the first work function metal from in between the two or morenanowires of the second stack. Further, the method includes removing themask and the plug from the first stack, and forming a second workfunction metal on the first stack and the second stack.

Embodiments of the invention are directed to a method for fabricating asemiconductor device. A non-limiting example of the method includesproviding a first stack and a second stack each having a first workfunction metal, where a mask covers the first stack and a plug isadjacent to both the mask and the first work function metal of the firststack. The method includes removing the first work function metal fromthe second stack, while the first work function metal of the first stackis protected by the mask and the plug. Also, the method includesremoving the mask and the plug, and forming a second work function metalon the first stack and the second stack.

Additional technical features and benefits are realized through thetechniques of the present invention. Embodiments and aspects of theinvention are described in detail herein and are considered a part ofthe claimed subject matter. For a better understanding, refer to thedetailed description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The specifics of the exclusive rights described herein are particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments of the invention are apparent from the followingdetailed description taken in conjunction with the accompanying drawingsin which:

FIG. 1A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 1B depicts a cross-sectional view of the semiconductor device shownin FIG. 1A taken along line A-A′ according to embodiments of theinvention;

FIG. 1C depicts a cross-sectional view of the semiconductor device shownin FIG. 1A taken along line B-B′ according to embodiments of theinvention;

FIG. 2A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 2B depicts a cross-sectional view of the semiconductor device shownin FIG. 2A taken along line A-A′ according to embodiments of theinvention;

FIG. 2C depicts a cross-sectional view of the semiconductor device shownin FIG. 2A taken along line C-C′ according to embodiments of theinvention;

FIG. 3A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 3B depicts a cross-sectional view of the semiconductor device shownin FIG. 3A taken along line A-A′ according to embodiments of theinvention;

FIG. 3C depicts a cross-sectional view of the semiconductor device shownin FIG. 3A taken along line C-C′ according to embodiments of theinvention;

FIG. 4A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 4B depicts a cross-sectional view of the semiconductor device shownin FIG. 4A taken along line A-A′ according to embodiments of theinvention;

FIG. 4C depicts a cross-sectional view of the semiconductor device shownin FIG. 4A taken along line C-C′ according to embodiments of theinvention;

FIG. 5A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 5B depicts a cross-sectional view of the semiconductor device shownin FIG. 5A taken along line A-A′ according to embodiments of theinvention;

FIG. 5C depicts a cross-sectional view of the semiconductor device shownin FIG. 5A taken along line C-C′ according to embodiments of theinvention;

FIG. 6A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 6B depicts a cross-sectional view of the semiconductor device shownin FIG. 6A taken along line A-A′ according to embodiments of theinvention;

FIG. 6C depicts a cross-sectional view of the semiconductor device shownin FIG. 6A taken along line C-C′ according to embodiments of theinvention;

FIG. 7A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 7B depicts a cross-sectional view of the semiconductor device shownin FIG. 7A taken along line D-D′ according to embodiments of theinvention;

FIG. 7C depicts a cross-sectional view of the semiconductor device shownin FIG. 7A taken along line C-C′ according to embodiments of theinvention;

FIG. 8A depicts a cross-fin middle-gate cross-sectional view of asemiconductor device after fabrication operations according toembodiments of the invention;

FIG. 8B depicts a cross-sectional view of the semiconductor device shownin FIG. 8A taken along line D-D′ according to embodiments of theinvention; and

FIG. 8C depicts a cross-sectional view of the semiconductor device shownin FIG. 8A taken along line C-C′ according to embodiments of theinvention.

The diagrams depicted herein are illustrative. There can be manyvariations to the diagram or the operations described therein withoutdeparting from the spirit of the invention. For instance, the actionscan be performed in a differing order or actions can be added, deletedor modified. Also, the term “coupled” and variations thereof describeshaving a communications path between two elements and does not imply adirect connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification.

In the accompanying figures and following detailed description of theembodiments of the invention, the various elements illustrated in thefigures are provided with two or three digit reference numbers. Withminor exceptions, the leftmost digit(s) of each reference numbercorrespond to the figure in which its element is first illustrated.

DETAILED DESCRIPTION

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

Turning now to an overview of technologies that are more specificallyrelevant to aspects of the invention, gate-all-around (GAA) nanosheetFET structures can provide superior electrostatics. In contrast to knownFin-type FET (FinFET) structures in which the fin element of thetransistor extends “up” out of the transistor, nanosheet FET designsimplement the fin as a silicon nanosheet/nanowire. In a knownconfiguration of a GAA nanosheet FET, a relatively small FET footprintis provided by forming the channel region as a series of nanosheets(i.e., silicon nanowires). A known GAA configuration includes a sourceregion, a drain region, and stacked nanosheet channels between thesource and drain regions. A gate surrounds the stacked nanosheetchannels and regulates electron flow through the nanosheet channelsbetween the source and drain regions. GAA nanosheet FETs are fabricatedby forming alternating layers of channel nanosheets and sacrificialnanosheets. The sacrificial nanosheets are released from the channelnanosheets before the FET device is finalized.

A dual work function metal scheme is difficult to realize for nanosheettechnology. A state-of-the-art method of forming dual work functionmetals is to protect the NFET (PFET) side by a mask layer (e.g., anorganic planarization layer (OPL)) and then etch away the work functionmetal on the PFET (NFET) side. However, this method does not work fornanosheets due to work function metal pinch-off between sheets.Completely etching the work function metal in between nanosheetsrequires a long etch time during which the etch can undercut below themask material and damage the work function metal on the protected device((NFET) or PFET).

Turning now to an overview of the aspects of the invention, one or moreembodiments of the invention address the above-described shortcomings ofthe prior art by providing a semiconductor device and a method offorming the semiconductor device in which a plug of liner is providedunderneath the masking layer. The plug protects the first work functionmetal on one side (e.g., the NFET side) of the semiconductor devicewhile the other side (e.g., the PFET side) is being processed. The firstwork function metal is removed from one side, and then the masking layerand plug are removed. The second work function metal can then be formedon both the NFET side and PFET side of the substrate/wafer.

Turning now to a more detailed description of aspects of the presentinvention, FIG. 1A depicts a cross-fin middle-gate cross-sectional viewof a semiconductor device 100 according to embodiments of the invention.FIG. 1B depicts a cross-sectional view of the semiconductor device 100taken along line A-A′ of FIG. 1A according to embodiments of theinvention. FIG. 1C depicts a cross-sectional view of the semiconductordevice 100 taken along line B-B′ of FIG. 1A according to embodiments ofthe invention. A replacement metal gate (RMG) process has beenperformed, as understood by one skilled in the art.

In FIG. 1A, the semiconductor device 100 has stacks 150 and 152, bottomisolation material 106 formed on a substrate 102, andnanowires/nanosheets 108A, 108B, and 108C. In each stack 150 and 152,the nanowires/nanosheets 108A, 108B, and 108C are separated by one ormore high-k materials 110 and a first work function metal 112. Thenanowires/nanosheets 108A, 108B, and 108C are wrapped in the high-kmaterial 110. The nanowires/nanosheets 108A, 108B, and 108C can bereferred to generally as nanowires/nanosheets. Although threenanowires/nanosheets 108A, 108B, and 108C are illustrated forexplanation purposes, the stacks 150 and 152 can each have two or morenanowires/nanosheets, such as 3, 4, 5, 6, etc.

Example materials for the nanosheet/nanowire layers 108 can includesilicon. The nanosheet layers 108 can be doped or undoped. When doped,the nanosheet/nanowire layers can include “P” type dopants such asboron, aluminum, gallium, and indium, or “N” type dopants such asphosphorus, arsenic, antimony. Other materials or combinations ofmaterials can also be used. Other non-limiting examples of semiconductormaterials for the nanosheet/nanowire layers 108 include strained Si, SiC(silicon carbide), Ge (germanium), SiGe (silicon germanium), SiGeC(silicon-germanium-carbon), Si alloys, Ge alloys, GaAs (galliumarsenide), InAs (indium arsenide), InP (indium phosphide), or anycombination thereof. In some embodiments of the invention, the thicknessof the nanosheet/nanowire layers 108 can be about 3 nm to about 20 nmthick. In some embodiments of the invention, the thickness of thenanosheet/nanowire layers 108 can be between about 10 nm to about 12 nm.Greater and smaller thicknesses are also contemplated.

The isolation material 106 can be, for example, an oxide material suchas silicon dioxide (SiO₂). The high-k dielectric materials 110 can be adielectric material having a dielectric constant greater than 4.0, 7.0,or 10.0. Non-limiting examples of suitable materials for the high-kdielectric material include oxides, nitrides, oxynitrides, silicates(e.g., metal silicates), aluminates, titanates, nitrides, or anycombination thereof. Examples of high-k materials include, but are notlimited to, metal oxides such as hafnium oxide, hafnium silicon oxide,hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide,zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride,tantalum oxide, titanium oxide, barium strontium titanium oxide, bariumtitanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide,lead scandium tantalum oxide, and lead zinc niobate. The high-k materialcan further include dopants such as, for example, lanthanum andaluminum.

The substrate 102 can encompasses semiconductor materials conventionallyused in the semiconductor industry from which to make electricaldevices. In embodiments of the invention, the starting substrate can bea semiconductor-on-insulator (SOI) substrate, which already includes theburied oxide layer. Alternatively, the starting substrate can be a bulksemiconductor including a sole semiconductor material or a combinationof two or more semiconductor materials. The semiconductor material caninclude one or more monocrystalline silicon materials, such as therelatively pure or lightly impurity-doped monocrystalline siliconmaterials typically used in the semiconductor industry, as well aspolycrystalline silicon materials, where silicon can be mixed with otherelements such as carbon and the like. Illustrative examples ofsilicon-containing materials suitable for the bulk-semiconductorsubstrate include, but are not limited to, silicon, silicon germanium,silicon germanium carbide, silicon carbide, polysilicon, epitaxialsilicon, amorphous silicon, and multi-layers thereof. The semiconductormaterial also includes other materials such as relatively pure andimpurity-doped gallium arsenide, germanium, gallium arsenide, galliumnitride, cadmium telluride, and zinc selenide, zinc oxide, glass, andthe like. The substrate 102 can be a monocrystalline silicon material.The silicon substrate 102 can be a bulk silicon wafer or can be a thinlayer of silicon disposed over an insulating layer (SOI) that, in turn,can be supported by a carrier wafer. The substrate 102 can be materialconsisting essentially of III-V compound semiconductors. Other suitablesubstrates can include II-VI compound.

The type of work function metal(s) depends on the type of transistor andcan differ between the NFET and the NFET. Examples of the first workfunction metal 112 and the second work function metal 802 depicted inFIGS. 8A, 8B, and 8C can be include p-type work function metal materialsand n-type work function metal materials. The first work function metal112 can be one type and the second work function metal 802 can beanother type. P-type work function materials include compositions suchas ruthenium, palladium, platinum, cobalt, nickel, and conductive metaloxides, conductive nitrides such as TiN, conductive carbide such as TiCor TiAlC, or any combination thereof. N-type metal materials includecompositions such as hafnium, zirconium, titanium, tantalum, aluminum,metal carbides (e.g., hafnium carbide, zirconium carbide, titaniumcarbide, and aluminum carbide), aluminides, conductive nitrides such asTiN, or any combination thereof.

FIG. 1B depicts an inter-layer dielectric (ILD) material 114 formed onthe bottom isolation material 106 and a trench 116 having a gate spacer118 on the sidewalls. The high-k material 110 is formed to line the ILDmaterial 114, the gate spacer 118, and the bottom isolation material 106at the bottom of the trench 116.

FIG. 1C depicts epitaxially grown source/drain regions 120A and 120Bconnected to opposite sides of the nanosheets 108 to form nanosheetchannels in between. Inner spacers 122 separate the nanosheets 108 atthe edges, while the high-k material 110 and first work function metal112 separate the nanosheets in the middle.

The ILD material 114 can be, for example, an oxide. A suitable a low-kdielectric material can be utilized for the ILD material 114. The gatespacer 118 can be, for example, an oxide or nitride material. The innerspacers 122 can be, for example, an oxide or nitride material.

Standard lithographic processes can be utilized to form the initialstructure in FIGS. 1A, 1B, and 1C as understood by one skilled in theart. An example process can include growing SiGe/Si multilayer materials(i.e., alternating sacrificial SiGe layers and Si nanosheet layers) on aSOI substrate, patterning the multilayer stack into fins, forming adummy gate structure, forming a spacer (e.g., gate spacer 118) on thedummy gate sidewall, directionally etching to remove stack materials inthe source/drain regions, forming inner spacers (e.g., as depicted inthe B-B′ cross-section of FIG. 1C, which can be done by first etchingback the exposed sacrificial SiGe and then filling in with the innerspacer material), and growing the source/drain epitaxial layer. Further,the example process can include depositing ILD and performing chemicalmechanical planarization/polishing, removing the dummy gate material,selectively removing the sacrificial SiGe to suspend the Si nanosheetchannel, and then depositing the high-k material and the first workfunction metal.

Terms such as “epitaxial growth” and “epitaxially formed and/or grown”refer to the growth of a semiconductor material on a deposition surfaceof a semiconductor material, in which the semiconductor material beinggrown has the same crystalline characteristics as the semiconductormaterial of the deposition surface. In an epitaxial deposition process,the chemical reactants provided by the source gases are controlled andthe system parameters are set so that the depositing atoms arrive at thedeposition surface of the semiconductor substrate with sufficient energyto move around on the surface and orient themselves to the crystalarrangement of the atoms of the deposition surface. Therefore, anepitaxial semiconductor material has the same crystallinecharacteristics as the deposition surface on which it is formed. Forexample, an epitaxial semiconductor material deposited on a {100}crystal surface will take on a {100} orientation.

FIG. 2A depicts a cross-fin middle-gate cross-sectional view of thesemiconductor device 100 according to embodiments of the invention. FIG.2B depicts a cross-sectional view of a semiconductor device 100 takenalong line A-A′ of FIG. 2A according to embodiments of the invention.FIG. 2C depicts a cross-sectional view of the semiconductor device 100taken along line C-C′ of FIG. 2A according to embodiments of theinvention. Deposition and patterning of masking material are performedusing standard lithographic processes.

A masking material or mask 202 is deposited on top of the semiconductordevice 100 such that the stacks 150 and 152 are covered. An example ofthe masking material 202 can be amorphous carbon. The masking material202 is patterned to leave one stack 150 covered while uncovering theother stack 152 as depicted in FIG. 2A. The masking material 202 can bepatterned using, for example, reactive ion etching (RIE). FIG. 2Bdepicts the masking material 202 covering first work function metal 112while the first work function metal 112 is exposed in FIG. 2C.

FIG. 3A depicts a cross-fin middle-gate cross-sectional view of thesemiconductor device 100 according to embodiments of the invention. FIG.3B depicts a cross-sectional view of a semiconductor device 100 takenalong line A-A′ of FIG. 3A according to embodiments of the invention.FIG. 3C depicts a cross-sectional view of the semiconductor device 100taken along line C-C′ of FIG. 3A according to embodiments of theinvention. An undercut is etched using standard lithograph processes.

An isotropic etch of the first work function metal 112 is performed totarget the work function metal width, so that an intentional overetchoccurs (during the isotropic etch) to create an undercut or gap 302underneath the masking material 202. The undercut or gap 302 is formedby removing a portion of the first work function metal 112 fromunderneath the masking material 202 so as to leave a gap. The thicknessor height (H) of the undercut/gap 302 corresponds to (i.e., matches) thethickness of the first work function metal 112 having been removed. Thethickness or height (H) of the undercut/gap 302 can be, for example, 3nm to 10 nm. The thickness or height (H) of the undercut/gap 302measured from the bottom surface of the making material 202 to the topsurface of the high-k material 110 underneath the masking material 202,where this high-k material 110 is depicted on top of the bottomisolation material 106 in FIG. 3A. High-k material 110 can be depositedby the so-called atomic layer deposition process which conformally coatsmaterial on every exposed surface. This undercut/gap 302 isintentionally formed in preparation for further processing, inaccordance with embodiments of the invention.

The first work function metal 112 can be etched using, for example, anisotropic etch. During the isotropic etch, FIG. 3A shows that the firstwork function metal 112 has also been etched back on the side of thestack 152, for example, 5 nm. Between the nanosheets 108 in stack 152,the first work function metal 112 has been etched back such that edgesof the nanosheets 108 extend or overhang over the missing area of thefirst work function metal 112 and such that spaces/areas 304 are formedat the recessed areas where the first work function metal 112 has beenremoved in FIG. 3A.

FIG. 3B depicts the undercut or gap 302 that runs underneath the maskingmaterial 202 after removal of a portion of the first work function metal112 by the isotropic etch. The undercut or gap 302 separates the maskingmaterial 202 from the high-k material 110, with a separation distance orheight (H) discussed in FIG. 3A. The view in FIG. 3C illustrates thatthe isotropic etch removes the first work function metal 112 such thatthe high-k material 110 is exposed.

FIG. 4A depicts a cross-fin middle-gate cross-sectional view of thesemiconductor device 100 according to embodiments of the invention. FIG.4B depicts a cross-sectional view of the semiconductor device 100 takenalong line A-A′ of FIG. 4A according to embodiments of the invention.FIG. 4C depicts a cross-sectional view of the semiconductor device 100taken along line C-C′ of FIG. 4A according to embodiments of theinvention. Pinch off is performed by liner deposition using standardlithographic processes.

A liner 402 is deposited to pinch off the undercut or gap 302 beneaththe masking material 202 as depicted in FIGS. 4A, 4B, and 4C. In otherwords, the liner 402 completely fills in the undercut or gap 302 beneaththe masking material 202 in a pinch off region in FIGS. 4A and 4B. Thepinch off region 404 is highlighted by a circle in FIG. 4A, while theview in FIG. 4B is the pinch off region 404 that has been filled in bythe liner 402. Between the nanosheets 108, the liner 402 is not pinchedoff (i.e., not completely filled) because of the larger gap between thenanosheets 108. The liner 402 can be a nitride, such as silicon nitride(SiN). Other example materials of the liner 402 can include SiCO, SiOCN,etc. A conformal deposition process is utilized to target the undercutor gap 302. For example, atomic layer deposition (ALD) can be used as aconformal deposition process among other types of conformal depositionprocesses.

FIGS. 4A and 4B illustrate the liner 402 filling in the previousundercut or gap 302 between the high-k material 110 and the maskingmaterial 202. Additionally, the liner 402 is on the top and sides of themasking material 202. On the side of the stack 152, FIGS. 4A and 4Cillustrate the liner 402 covering the high-k material 110.

FIG. 5A depicts a cross-fin middle-gate cross-sectional view of thesemiconductor device 100 according to embodiments of the invention. FIG.5B depicts a cross-sectional view of the semiconductor device 100 alongline A-A′ of FIG. 5A according to embodiments of the invention. FIG. 5Cdepicts a cross-sectional view of the semiconductor device 100 alongline C-C′ of FIG. 5A according to embodiments of the invention. Removalof a portion of the liner 402 is performed using standard lithographicprocesses.

The liner 402 is etched back by conformal etching such that the pinchoff region 404 remains to protect the left stack 150 from a subsequentetch to the first work function metal 112 as depicted in FIGS. 5A and5B. The remaining liner 402 is or functions as a plug 402A at the end ofthe first work function metal 112 as seen underneath the maskingmaterial 202 in the pinch off region 404. The liner 402 is etched backto (substantially) align with the edge of the masking material 202 suchthat the first work function metal 112 is not exposed; this means thatsubsequent etching and fabrication processing does not reach ordeteriorate the protected first work function metal 112 via any undercutor gap. In some implementations, the etched back liner 402 might notexactly align with the edge of the masking material 202 and a smalloveretch is acceptable.

The liner 402 is removed from the top and sides of the masking materialand from the stack 152 on the right side of the semiconductor device100, while the liner 402 (i.e., plug 402A) at the pinch off region 404remains intact. Accordingly, the first work function metal 112 isexposed in the stack 152 on the right side of the semiconductor device100 in FIG. 5A. FIG. 5B illustrates that the liner 402 remains beneaththe masking material 202. FIG. 5C is a view illustrating areas notprotected by the liner 402 has been removed.

FIG. 6A depicts a cross-fin middle-gate cross-sectional view of thesemiconductor device 100 according to embodiments of the invention. FIG.6B depicts a cross-sectional view of a semiconductor device 100 alongline A-A′ of FIG. 6A according to embodiments of the invention. FIG. 6Cdepicts a cross-sectional view of the semiconductor device 100 alongline C-C′ of FIG. 6A according to embodiments of the invention.

An aggressive etch is utilized to remove exposed first work functionmetal 112 between the nanosheets 108 as depicted in FIG. 6A. The exposedfirst work function metal 112 can be etched using, for example, anhydrofluoric acid (HF) based wet etchant. During the etch of the exposedfirst work function metal 112 in stack 152, the plug 402A of liner 402,remaining at the first work function metal end (i.e., at the pinch offregion 404) underneath the masking material 202, is designed to preventan undesired undercut as depicted in FIGS. 6A and 6B. This liner plug402A protects the first work function metal 112 under the maskingmaterial 202 on the left side of the semiconductor device 100, whichconcurrently protects the stack 150. Although the right stack 152 ofnanosheets 108 encompassed by the high-k material 110 appears to befloating in air, it is noted that FIG. 1C shows that the source/drainregions 120A and 120B and inner spacers 122 are actually holding thenanosheets 108 in the right stack 152 in place, as understood by oneskilled in the art. The view in FIG. 6C has no first work function metalto remove.

FIG. 7A depicts a cross-fin middle-gate cross-sectional view of thesemiconductor device 100 according to embodiments of the invention. FIG.7B depicts a cross-sectional view of a semiconductor device 100 alongline A-A′ of FIG. 7A according to embodiments of the invention. FIG. 7Cdepicts a cross-sectional view of the semiconductor device 100 alongline C-C′ of FIG. 7A according to embodiments of the invention.

The masking material 202 and remaining liner 402 (i.e., the liner plug402A) at the end of the first work function metal 112 are removed. Ascan be seen in FIGS. 7A and 7B, the masking material 202 no longercovers the first work function metal 112. The right side (including theright stack 152) of the semiconductor device 100 has no first workfunction metal 112 as depicted in FIGS. 7A and 7C. Both the left stack150 and right stack 152 are ready for deposition of a second workfunction metal discussed below.

FIG. 8A depicts a cross-fin middle-gate cross-sectional view of thesemiconductor device 100 according to embodiments of the invention. FIG.8B depicts a cross-sectional view of a semiconductor device 100 alongline A-A′ of FIG. 8A according to embodiments of the invention. FIG. 8Cdepicts a cross-sectional view of the semiconductor device 100 alongline C-C′ of FIG. 8A according to embodiments of the invention.

FIGS. 8A, 8B, and 8C illustrate deposition of a second work functionmetal 802 on the semiconductor device 100. On the left side of FIG. 8A,the left stack 150 includes the second work function 802 formed directlyon top of the first work function metal 112, while the first workfunction metal 112 is formed directly on the high-k material 110surrounding the nanosheets 108. A step 804 is formed at the previouslocation of the pinch off 404.

On the right side of FIG. 8A, the right stack 152 includes the secondwork function 802 formed directly on the high-k material 110 surroundingthe nanosheets 108.

For the left stack 150, its gate stack includes the high-k dielectricmaterial 110, the first work function metal 112, and the second workfunction metal 802, where the first and second work function metals aredifferent. For the right stack 152, its gate stack includes the high-kdielectric material 110 and the second work function metal 802.

Contacts (not shown) can be formed to provide electrical access to thegate stacks and the source/drain regions 120A and 120B for operation ofthe transistor, as understood by one skilled in the art.

According to embodiments of the invention, a method of fabricating asemiconductor device 100 is provided. The method includes forming afirst work function metal 112 in a first stack 150 and a second stack152, the first stack and the second stack each having two or morenanowires 108 (e.g., nanowires 108A, 108B, 108C) separated by the firstwork function metal 112. The method includes forming a mask 202 on thefirst stack 150 such that the first work function metal 112 in the firststack 150 is protected while the first work function metal 112 in thesecond stack 152 is exposed (i.e., not protected by the mask 202). Themethod includes undercutting the mask 202 by removing a portion of thefirst work function metal 112 in the first stack 150, such that a gap302 remains at a location where the portion is removed and forming aplug 402A in the gap 302 underneath the mask 202 at the location so asto protect the first work function metal in the first stack 150. Themethod includes removing the first work function metal in the secondstack 152, thereby removing the first work function metal 112 from inbetween the two or more nanowires 108 of the second stack 152. Themethod includes removing the mask 202 and plug 402A from the first stack150 and forming a second work function metal 802 on the first stack 150and the second stack 152.

The second work function metal 802 is formed on top of the first workfunction metal 112 in the first stack 150. No second work function metalis formed in between the two or more nanowires 108 of the first stack150. Forming the second work function metal 802 on the first stack 150and the second stack 152 causes the second stack 152 to have the two ormore nanowires 108 separated by the second work function metal 802.

Undercutting the mask 202 by removing the portion of the first workfunction metal 112 in the first stack 150 includes etching the firstwork function metal 112 which removes the portion thereby creating anundercut below the mask 202 on the first stack 150. Etching the firstwork function metal 112 creates recessed areas in the first workfunction metal 112 in the second stack 150 as depicted in FIG. 5A. Thetwo or more nanowires 108 overhang the recessed areas of the first workfunction metal 112 in the second stack 150 as depicted in FIG. 5A.

Forming the plug 402A in the gap 302 underneath the mask 202 at thelocation so as to protect the first work function metal 112 in the firststack includes depositing a liner 402 such that the liner pinches off inthe gap 302 as depicted in FIGS. 4A, 4B, 4C. The liner 402 is removedexcept for in the gap 302, such that the liner 402 remaining in the gapforms the plug 402A. The liner covers the mask 202. The liner 402 coversthe second stack 152. The plug 402A is at an end of the first workfunction metal 112 in the first stack 150. The plug 402A is formed of anitride material.

According to embodiments of the invention, a method of fabricating asemiconductor device 100 is provided. The method includes providing afirst stack 150 and a second stack 152 each having a first work functionmetal 112, where a mask 202 covers the first stack 150 and a plug 402Ais adjacent to both the mask 202 and the first work function metal 112of the first stack 150. The method includes removing the first workfunction metal 112 from the second stack 152, while the first workfunction metal 112 of the first stack 150 is protected by the mask 202and the plug 402A. The method includes removing the mask 202 and theplug 402A and forming a second work function metal 802 on the firststack 150 and the second stack 152.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layer “C”) isbetween layer “A” and layer “B” as long as the relevant characteristicsand functionalities of layer “A” and layer “B” are not substantiallychanged by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “exemplary” is used herein to mean “serving as anexample, instance or illustration.” Any embodiment or design describedherein as “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements.

The phrase “selective to,” such as, for example, “a first elementselective to a second element,” means that the first element can beetched and the second element can act as an etch stop.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

As previously noted herein, for the sake of brevity, conventionaltechniques related to semiconductor device and integrated circuit (IC)fabrication may or may not be described in detail herein. By way ofbackground, however, a more general description of the semiconductordevice fabrication processes that can be utilized in implementing one ormore embodiments of the present invention will now be provided. Althoughspecific fabrication operations used in implementing one or moreembodiments of the present invention can be individually known, thedescribed combination of operations and/or resulting structures of thepresent invention are unique. Thus, the unique combination of theoperations described in connection with the fabrication of asemiconductor device according to the present invention utilize avariety of individually known physical and chemical processes performedon a semiconductor (e.g., silicon) substrate, some of which aredescribed in the immediately following paragraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), andchemical-mechanical planarization (CMP), and the like. Semiconductordoping is the modification of electrical properties by doping, forexample, transistor sources and drains, generally by diffusion and/or byion implantation. These doping processes are followed by furnaceannealing or by rapid thermal annealing (RTA). Annealing serves toactivate the implanted dopants. Films of both conductors (e.g.,poly-silicon, aluminum, copper, etc.) and insulators (e.g., variousforms of silicon dioxide, silicon nitride, etc.) are used to connect andisolate transistors and their components. Selective doping of variousregions of the semiconductor substrate allows the conductivity of thesubstrate to be changed with the application of voltage. By creatingstructures of these various components, millions of transistors can bebuilt and wired together to form the complex circuitry of a modernmicroelectronic device. Semiconductor lithography is the formation ofthree-dimensional relief images or patterns on the semiconductorsubstrate for subsequent transfer of the pattern to the substrate. Insemiconductor lithography, the patterns are formed by a light sensitivepolymer called a photo-resist. To build the complex structures that makeup a transistor and the many wires that connect the millions oftransistors of a circuit, lithography and etch pattern transfer stepsare repeated multiple times. Each pattern being printed on the wafer isaligned to the previously formed patterns and slowly the conductors,insulators and selectively doped regions are built up to form the finaldevice.

The flowchart and block diagrams in the Figures illustrate possibleimplementations of fabrication and/or operation methods according tovarious embodiments of the present invention. Variousfunctions/operations of the method are represented in the flow diagramby blocks. In some alternative implementations, the functions noted inthe blocks can occur out of the order noted in the Figures. For example,two blocks shown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A semiconductor device comprising: a first stackcomprising a first two or more nanowires, a first work function metal onthe first two or more nanowires, and a second work function metal on thefirst work function metal, wherein the first two or more nanowires areseparated by the first work function metal; and a second stackcomprising a second two or more nanowires and the second work functionmetal, wherein the second two or more nanowires are separated by thesecond work function metal.
 2. The semiconductor device of claim 1,wherein the second work function metal is formed on top of the firstwork function metal in the first stack.
 3. The semiconductor device ofclaim 1, wherein the second work function metal is not formed in betweenthe first two or more nanowires in the first stack.
 4. The semiconductordevice of claim 1, wherein a dielectric material is formed around thefirst two or more nanowires.
 5. The semiconductor device of claim 4,wherein the dielectric material comprises a dielectric constant greaterthan 4.0.
 6. The semiconductor device of claim 4, wherein the dielectricmaterial comprises a dielectric constant greater than 7.0.
 7. Thesemiconductor device of claim 4, wherein the dielectric materialcomprises a dielectric constant greater than 10.0.
 8. The semiconductordevice of claim 1, wherein a dielectric material is sandwiched betweenthe first work function metal and the first two or more nanowires. 9.The semiconductor device of claim 1, wherein a dielectric material isformed around the second two or more nanowires.
 10. The semiconductordevice of claim 9, wherein the dielectric material comprises adielectric constant greater than 4.0.
 11. The semiconductor device ofclaim 9, wherein the dielectric material comprises a dielectric constantgreater than 7.0.
 12. The semiconductor device of claim 9, wherein thedielectric material comprises a dielectric constant greater than 10.0.13. The semiconductor device of claim 1, wherein a dielectric materialis sandwiched between the first work function metal and the second twoor more nanowires.
 14. The semiconductor device of claim 1, wherein thefirst work function metal and the second work function metal aredifferent.
 15. The semiconductor device of claim 1, wherein the firstwork function metal comprises a p-type work function metal and thesecond work function metal comprises an n-type work function metal. 16.The semiconductor device of claim 1, wherein the first work functionmetal comprises an n-type work function metal and the second workfunction metal comprises a p-type work function metal.
 17. Thesemiconductor device of claim 1, wherein a step is between the firststack and the second stack.
 18. The semiconductor device of claim 17,wherein the step is formed by the second work function metal in thefirst stack overlapping the second work function metal in the secondstack.
 19. The semiconductor device of claim 1, wherein the first stackand the second stack are formed on a substrate.
 20. The semiconductordevice of claim 19, wherein isolation material separates the first stackand the second stack from the substrate.